(1) Field of the Invention
The present invention relates to novel heat stable, mesoporous to small macroporous, inorganic silica foam compositions. In particular, the present invention relates to porous silica foam compositions which are highly cross-linked having a ratio of Q4/(Q3+Q2) of between 2.5 and 8.
(2) Description of Related Art
Mesoporous cellular foams (MCF) with large pore sizes (20 to 45 nm) are relatively new in the art, being templated by microemulsion of xe2x80x9coil in waterxe2x80x9d. Foams of this type are described by Schmidt-Winkel et al in J. Am. Chem. Soc. 121 254-255 (1999) and Chem. Materials 12 686-696 (1999). According to the teachings provided by Schmidt-Winkel et al, MCF materials exhibit x-ray diffraction peaks at small scattering angles. However, the peaks cannot be indexed to any plane or space group indicative of a regular ordered structure. Instead, the x-ray peaks are consistent with the presence of cells of more or less spherical shape and size. They further teach that direct evidence for a cellular foam structure is obtained from transmission electron microscopy (TEM) images which show a reticulated assembly of cells connected by open windows with an average diameter smaller than the average diameter of the cells. The silica walls of the cells are described as being xe2x80x9cstrut-likexe2x80x9d.
The prior art foams are prepared from microemulsion solution of BASF PLURONIC 123(trademark), a poly(ethylene oxide) block-(polypropylene oxide) block-(poly(ethylene oxide) block triblock co-polymer (EO20PO70EO20), Molecular Weight Average=5800 (Aldrich, Milwaukee, Wis.), and an organic co-solvent (1,3,5-trimethylbenzene) in the presence of a strong acid (HCl) admixed with tetraethoxysilane as the silica forming agent. The resulting very acidic conditions limited the silica framework crosslinking to a maximum Q4/(Q2+Q3) value of 2.5 wherein Qn is the fraction of tetrahedral SiO4 centers linked to n adjacent silicon atoms through bridging oxygens. In most silica compositions, the magnitude of Q2 is small in relationship to the values of Q3 and Q4, and not readily observable by 29Si MAS NMR, so that the crosslinking parameter is approximately equal to the ratio Q4/Q3. The Q4 and Q3 environments are readily distinguished by resolvable 29Si MAS NMR lines, the relative intensities of which provide the magnitudes of Qn. The optimal crosslinking value of 2.5 was obtained under conditions that require the presence of fluoride ion to facilitate framework crosslinking and optimize the pore size. In the absence of fluoride ion the value of the crosslinking parameter was 2.22. The relatively low framework crosslinking parameter, Q4/Q3+Q2, limits the framework stability under hydrothermal conditions. Also, the use of highly acidic reaction conditions and fluoride ion are undesirable because of the corrosive nature of these reagents.
There is a need for porous silica foam compositions with improved framework crosslinking and stability properties. There is also a need for foams which are economical to prepare under less corrosive conditions. Also, it is desirable for applications of MCF materials in thermal insulation to increase the pore size distribution to include the small macropore cell size region from 50-100 nm. These and other objects will become increasingly apparent by reference to the following description and the drawings.
The present invention relates to a mesoporous to small macroporous cellular silica foam composition with interconnected cells joined at nexus defining cellular pores with open windows between the cellular pores and with the SiO4 tetrahedra crosslinked to four adjacent silicon sites (Q4), to three adjacent silicon sites (Q3), and to two adjacent silicon sites (Q2), the ratio of Q4/(Q3+Q2) being between 2.5 and about 8.
The present invention also relates to a hybrid mesoporous to small macroporous cellular silica foam composition with interconnected cells joined at nexus defining cellular pores with open windows between the cellular pores and with SiO4 tetrahedra of the cell walls crosslinked to four adjacent silicon sites (Q4), to three adjacent silicon sites (Q3), and to two adjacent silicon sites (Q2), the ratio of Q4/(Q3+Q2) being between 2.5 and about 8, containing a surfactant and an organic co-solvent which swells the surfactant in the cellular pores.
Further, the present invention relates to a process for the preparation of a mesoporous to small macroporous cellular silica foam composition with interconnected cells joined at nexus which comprises:
(a) providing an aqueous mixture of a surfactant and an organic co-solvent which swells the surfactant as emulsifying agents;
(b) providing a solution of a water soluble silicate;
(c) providing an acid in an amount sufficient to cause precipitation of silica from the silicate solution at a pH between about 5.0 and 9.0.
(d) combining the reagents in parts (a), (b) and (c) at a temperature greater than xe2x88x9220xc2x0 C.;
(e) allowing the reaction mixture of step (d) to age for a minimum time of 5 minutes at one or more temperatures above xe2x88x9220xc2x0 C.; and
(f) recovering the precipitated product from the solution.
Preferably the soluble silica solution is a sodium silicate with SiO2/OHxe2x88x92 ratio of between 0.7 and 2.0.
The process also optionally includes removing the surfactant and the co-solvent by solvent extraction or calcination or by a combination of solvent extraction and calcination of the precipitated product in air at a temperature greater than 300xc2x0 C. for not less than 30 minutes. At least 85% of the surfactant and organic co-solvent can be removed from the cells by extraction with boiling ethanol, as well as other hot organic solvents, but it is generally more convenient to remove the surfactant and organic co-solvent by calcination.
The silica foam composition has pores in the mesopore range (2-50 nm) and into the small macropore range (50-100 nm), preferably has pores between 10 and 100 nm in diameter and windows between the cells with a diameter between about 5 and 70 nm. The volume per unit weight is between about 1 and 4 cc per gram. Most importantly, the compositions have cell walls wherein the SiO4 crosslinking parameter       Q    4              Q      2        +          Q      3      
has a value of at least 2.5 in the absence of fluoride and most preferably, values greater than this minimum value, in order to improve the thermal and hydrothermal stability of the cell walls. Kim et al (Science 282 1302 (1998)) have demonstrated that the thermal and hydrothermal stability of a porous silica structure is substantially improved by increasing the crosslinking of the SiO4 tetrahedra that comprise the pore walls. A framework crosslinking parameter, Q4/Q3+Q2, with a value of at least 2.5 and up to about 8 is possible by digestion of as-made mesostructures at temperatures of 100xc2x0 C. and above. The exact value of the crosslinking parameter is determined by the digestion temperature and time. In general, the value of the crosslinking parameter increases with increasing digestion temperature. The importance of a high SiO4 crosslinking value cannot be overstated, because the utility of porous silica compositions is highly dependent on structural stability under thermal and hydrothermal conditions. The novelty of the present invention lies in part on providing silica foam compositions with SiO4 crosslinking parameter values substantially greater than the limiting maximum value of 2.5 of previously reported foam compositions. Also, the expanded cellular pore size distribution, which includes small macropores in the range 50-100 nm, is another novel feature of this invention.
By incorporating other inorganic elements along with silicon into the assembly of the porous cellular foams, inorganic functionality can be introduced into the foam compositions. Many metallic elements (e.g. P, Ba, Y, La, Ce, Sn, Ga, Zn, Co, Ni, Co, Mo, and Cu) can be incorporated into the silica walls of the foams by linking to oxygen atoms in the walls. Aluminum, titanium, vanadium and the other said elements are easily incorporated into silica frameworks simply by including reagents of these elements into the foam assembly reaction.
In addition, organic or organometallic functionality can be introduced into the foam compositions of this invention by reacting the said compositions with silane coupling agents selected from the group X3SiR, X2SiR2, XSiR3 and mixtures thereof where X is a hydrolyzable group (e.g. alkoxide or halide) and R is an organo group of the desired functionality.